The new laser-based ALIMA measuring system during a test in front of the DLR hangar in Oberpfaffenhofen. ALIMA uses light detection and ranging (lidar), which, like radar, detects the backscattering of signals, in this case transmitted using a high-performance pulsed laser. This technique allows gravity waves to be detected at altitudes of up to 90 kilometres during flight.

Scientists from DLR, KIT, Forschungszentrum Jülich and the universities of Mainz and Frankfurt are coordinating the measurement campaign.

The influence of gravity waves on polar vortices over Antarctica, and chemical and dynamic processes in the tropopause region are being analysed.

Focus: Aeronautics, climate change

The German High Altitude and Long Range (HALO) research aircraft will be exploring the atmosphere in the southern hemisphere and its impact on climate change during September and November 2019 as part of the SOUTHTRAC (Transport and Composition of the Southern Hemisphere UTLS) mission. The main objective of the first phase of the campaign is to investigate gravity waves at the southern tip of South America and over Antarctica. In the second phase of the campaign, in November, the scientific focus of the investigations will shift to the exchange of air masses between the stratosphere and troposphere. During the transfer flights between Europe and South America, the researchers will investigate how current burning of biomass in the Amazon rainforest is affecting the climate, among other things. Scientists from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR), the Karlsruhe Institute of Technology (KIT), Forschungszentrum Jülich (FZJ) and the universities of Mainz and Frankfurt will coordinate the extensive research flights.

Trace gases, such as ozone and water vapour, are effective greenhouse gases and play an important role in climate change. Since the late 1980s the Montreal Protocol has restricted the manufacture of substances such as hydrochlorofluorocarbons, which severely deplete the ozone layer. A very large hole has formed in the ozone layer over the Antarctic region. It will take many decades for the ozone layer to fully recover. The importance of this for climate change in the southern hemisphere is now being investigated in detail by researchers as part of the SOUTHTRAC campaign.

The most important atmospheric conditions for the formation of the ozone hole over Antarctica are low temperatures and the reduced exchange of air masses with mid-latitudes. The latter is ensured by a stable Antarctic polar air vortex, but this can be weakened by strong wave activity. “For the first time in this region, we are investigating the excitation and propagation of gravity waves into the middle atmosphere at an altitude of 90 kilometres, which is partly triggered by the airflow over the Andes and the Antarctic peninsula. These slow down the polar vortex,” says Markus Rapp, Director of the DLR Institute of Atmospheric Physics. “Until now, this effect has not been sufficiently taken into account in climate and weather models.”

“During the measurement flights, we also want to examine the chemical and dynamic processes that influence trace gases like ozone and water vapour in the tropopause region,” says Björn-Martin Sinnhuber of the Institute of Meteorology and Climate Research – Atmospheric Trace Gases and Remote Sensing at KIT. “We will also look at the role played by biomass combustion of the kind that is currently occurring with the fires in the Amazon rainforest.” The universities of Wuppertal and Heidelberg are also partners in the project.

Innovative remote sensing techniques will be combined with highly accurate local measurements on the aircraft and compared with satellite data. “In order to adapt the flights optimally to the meteorological situation, atmospheric modellers will be on site, using predictions made by the Jülich atmosphere model CLaMS (Chemical Long-Range Model of the Stratosphere),” says Martin Riese, Deputy Director of the Institute of Energy and Climate Research at FZJ. The long-range airborne measurements conducted using HALO will be supplemented by activities conducted on the ground. Balloon radiosondes will be launched, and measurements will be performed on board a glider operating from the city of El Calafate. Meteorological and chemical forecasting models will provide the information about the local weather, atmospheric circulation and trace gas distribution required for precise flight planning.

Airborne laser looking upwards

The DLR team for the SOUTHTRAC mission will be using laser measurement technology on board HALO and in a measurement station on the ground in order to record gravity waves as far up as the middle atmosphere. This uses light detection and ranging (lidar), which, like radar, detects the backscattering of signals, in this case transmitted using a high-performance pulsed laser. “For the first time, HALO is using the Airborne LIdar for Middle Atmosphere research (ALIMA) to explore altitudes between 15 and 90 kilometres. This is the region where gravity waves influence the global circulation system, and has not yet been given adequate consideration in climate models,” says Bernd Kaifler, who, together with his colleagues at the DLR Institute of Atmospheric Physics, was responsible for the development and construction of the unique new instrument. The spectral measurement and evaluation of the laser light backscattered in the atmosphere and collected by a telescope on board HALO provides the researchers with high-resolution temperature, wind and aerosol profiles along the flight path, and therein the signatures of the gravity waves.

Oceans, mountains, gravity waves
Gravity waves form when atmospheric circulation systems are disrupted. They are observed as periodic fluctuations in temperature, pressure and wind that can extend up to 90 kilometres high in the middle atmosphere – the stratosphere and the mesosphere. They are triggered when strong wind systems meet high mountains. With mountains ranges running from north to south, which present a major obstacle to the very strong winds at these latitudes, the southern tip of South America and the Antarctic peninsula make an ideal place to investigate the lifecycle of these waves and their impact on climate change in the southern hemisphere.

Night shifts for research

The research flights will take place exclusively at night in order to ensure that the laser measurements are as disturbance-free as possible. “The numerous night flights represent a challenge for our crew,” explains DLR research pilot Marc Puskeiler. “Flights to Antarctica, in particular, where there is no air traffic control and no alternative airfields, have to be very carefully planned if they are to return safely to their starting point in Rio Grande, Argentina.” HALO was flown here in three stages from its home base at the DLR site in Oberpfaffenhofen, near Munich. It first travelled to the Cape Verde Islands and then onward to Buenos Aires.

About HALO

The High Altitude and Long Range (HALO) research aircraft is a joint initiative of German environmental and climate research institutions. HALO is supported by grants from the Federal Ministry of Education and Research (BMBF), the German Research Foundation (DFG), the Helmholtz Association of German Research Centres, the Max Planck Society (MPG), the Leibniz Association, the Free State of Bavaria, the Karlsruhe Institute of Technology (KIT), the Forschungszentrum Jülich and the German Aerospace Center (DLR).